1. Field of the Invention
The present invention relates to a numerical control device with a function of estimating the lifetime of a bearing supporting a spindle of a machine tool.
2. Description of the Related Art
A bearing that supports the spindle of a machine tool is gradually degraded by friction or load, so that it should be replaced with a new one at an appropriate time. Conventionally, a total number of revolutions and total rotation time of the spindle, which are obtained by integrating the rotational speed and rotation time of the spindle, are used as criteria for the time to replace the spindle bearing of the machine tool. For example, a numerical control device with a lifetime management function is disclosed in Japanese Patent JP5-208343A. This numerical control device determines the lifetime of a movable part by comparing a cumulative value of relevant operational quantities and a preset life value of the movable part, and comprises display means for displaying the cumulative value. Further, a bearing life monitoring method is disclosed in Japanese Patent JP2011-247660A. According to this monitoring method, the life value of a bearing is obtained by individually calculating average values of the rotation numbers of a rotor supported by the bearings and loads on the rotor, based on the operating conditions of a machine. The maintenance timing is displayed as a message when the lifetime is exceeded by an integrated value of a rotation time of the rotor.
The life of the bearing that supports the spindle of the machine tool is affected by a force that acts on the spindle. The spindle and a workpiece are not in contact with each other when the workpiece is not being cut. When the workpiece is being cut, however, the spindle contacts the workpiece, so that a force equivalent to a reaction force from the workpiece acts on the spindle. In the cutting mode, therefore, a load on the bearing is greater than in the non-cutting mode, so that the life of the bearing becomes shorter.
Conventionally, however, the rotational speed and rotation time of the spindle are simply integrated irrespective of the mode, cutting or non-cutting, so that the influence of the load on the spindle is ignored. According to the lifetime management technique disclosed in Japanese Patent JP5-208343A, for example, the operational quantities of the spindle are simply integrated without consideration of the influence of the force on the movable part. If the predetermined cumulative value is reached, therefore, the movable part is inevitably determined to require replacement without regard to the magnitude of the force thereon.